The paralleloscope weighed 17 lb and consisted of a 33 1/4- x 5 1/2- x 3 1/2-in. case in which was mounted a 90 deg prism (30 x 2 1/16 x 1 1/32 in.). The prism was made of a low grade of glass, containing imperfections such as bubbles, seeds, and striae. The image quality of the prism was extremely poor. The resolving power was 129 sec, measured at the extremes of the prism and at the center. The theoretical resolving power for such a prism of 1.25-in. aperture would be approximately 4.4 sec. Astigmatism was evident in the horizontal meridian, and image doubling was noted. However, the intended performance of the instrument will not be seriously affected by these factors. It was concluded that a similar instrument could be designed which would have as its principal component a series of plane mirrors of prisms rigidly mounted and that the design could be such that the instrument would fold up when not in use.